Secondary electron emitter and process of preparing same



March 5, 1957 P. RAPPAPORT 2,784,123 SECONDARY ELECTRON EMITTER AND PROCESS oF PREPARING SAME:

Filed May 1, 1952 ,4g/7g Amay" TTOR NE Y United States Patent SECONDARY ELECTRON EMITTER AND PROCESS F PREPARING SAME.

Paul Rappaport, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 1, 1952, Sen'al No. 285,466 14 Claims. (Cl. 14S- 6.3)

This invention relates to Ian improved method of processing a secondary emitter electrode for use in an electron multiplier type electron tube, and to secondary electron emitters made by this process.

In the drawing, Figs. 1, 2 and 3 schematically show the various steps in the process of activating a AgMg alloy secondary emitter according to the invention and Fig. 4 is a graph showing the comparison of tests of secondary emission ratio of a AgMg secondary emitter processed according to the invention with a AgMg emitter processed by the previously used method.

In general, the best secondary emitters are those that consists of an oxide film on a metal base member. One of the most satisfactory ways to achieve this is by the use of a AgMg alloy of approximately 98.3 percent silver and 1.7 percent magnesium as the base member. A magnesium oxide film is formed on this alloy base member when it is heated for a suitable time in an oxidizing atmosphere. The magnesium atoms inside the AgMgalloy move around due to the increased temperature, and those arriving at the surface of the alloy become oxidized and stick, thus forming the MgO lm. This type of surface has a number of d-istinct advantages over other secondary emitters. Some of these advantages are: (1) High secondary emission ratios that remain constant for several thousand hours under the conditions prevailing in standard receiving tubes; (2) Low resistance surface and interface so that arcing and Malter effect does not occur, thus giving good high frequency response; (3) Ease and uniformity of activation once the proper parameters are set; (4) No deterioration upon a few hours exposure to air of activated samples; and (5) No further activating required after the emitter is placed in vacuum. ri`he main disadvantage of this type of emitter as heretofore made is its inability to withstand heat without appreciable magnesium evaporation, during activation and operation of an electron multiplier tube in which the emitter is employed.

The object of the present invention is to provide oxidized silver-magnesium alloy emitters which are not subject to magnesium evaporation.

Throughout the specification and claims, the term oxygem when used alone, refers to oxygen gas. Thus, an oxygen-free atmosphere is a gas medium containing no appreciable oxygen gas.

Normally, it would be expected that the proper method for activating a AgMg alloy base member would be to bake it in an oxygen atmosphere. Some of the early workers were able to do this successfully when using high frequency induction heating and low oxygen pressures, but, in general, variable results were obtained and no completely reproducible technique was evolved.`

Experience shows that baking in oxygen at pressures of from a few millimeters mercury to atmospheric'pressure is not the best method of processing AgMg secondary emitters. when very little oxygen is present.

In fact, the best activation of AgMg results The most reliable and reproducible technique up to now has been developed by Dr. C. W. Mueller. This technique consists of baking the clean alloy base member for approximately 1/2 hour at a temperature of 550 C. in a vacuum system which rst has water vapor added and is then pumped down with a mechanical pump and a mercury diffusion pump. Dry Icey and acetone used as a cold trap which provides a residual water vapor pressure of about 5X [0*4 min/Hg. This technique produces oxidized AgMg. alloy emitters having a MgO surface about 1000 angstrom units thick, with a characteristic golden color and good secondary emitting roperties. lf a liquid air trap is used on the system, and no water vapor is added, the residual water vapor pressure can be as low as 10-22 mm./Hg and no noticeable change in the alloy takes place when baking except for a general outgassing and cleaning. However, if the alloy is baked in a commercial dry hydrogen furnace Where the probability of finding oxygen is very small, one still gets acceptably activated emitters, probably'dueV to the 8 i03 min/Hg water Vapor pressure present.

ln order to understand what is taking place, we must consider the relative diffusion rates of the Various components in the silver magnesium alloy. Free magnesium diffuses readily through the alloy when it is heated over 450 C., but not nearly as fast as does oxygen. It isy well known that silver when hot can pass copious quan tities of oxygen, and the small addition of magnesium to the silver apparently does not affect this phenomenon very much. Magnesium oxide and water vapor molecules do not diffuse through the alloy to any appreciable extent, because of their relatively large size. Experiments have shown that both oxygen and magnesium will diffuse through the MgO surface which is formed on alloy base member. Whether or not this is due' to cracks and pin holes in the surface is not certain.

ecause of the above reasons, when the AgMg alloy' base member is initially baked in an oxygen atmosphere, the oxygen diffuses into the alloy much faster than the Mg can diffuse to the surface. As the oxygen penetrates the alloy, it combines with any magnesium atomsV it contacts and the immobile MgO molecules formed are frozen in place with the alloy, thus interfering with' the formation of a surface MgO film. Attempts to acti vate further an oxygen-baked sample by the water vapor method gave no oxide film over the free silver areas, thus showing the lack of free magnesium after the xygen bake.

However, when the alloy base member is initially baked in a substantially oxygen-free atmosphere including some other molecular oxidizer, such as water vapor, which does not diffuse into the alloy to any appreciable extent, the magnesium is allowed to diffuse to the surface before it is oxidized. The magnesium arriving at the surface forms the oxide due to the action of water vapor and heat. The process can take place in: a vacuum chamber containing a residual pressure of water vapor, as described above; a hydrogen atmosphere with residual water vapor; or an inert gas, such as helium, at atmospheric pressure containing a low partial pressure of water vapor. Instead of water vapor, other molecular oxidizing agents which will not diffuse freely into the AgMg alloy, such as alcohols, carbon dioxide and nitrogen pentoxide, may be used.

Since the molecular oxidizing agent cannot freely dif# fuse into the alioyfany magnesium that has -not arrivedl at the surface during the baking processit still present` in the metallic form in the alloy the emitter is later operated at high temperatures, asin the RF heating during outgasing and cathode breakdown,

or by dynode power dissipation during use'in an electroni` The evaporation of magnesium can cause shorts,

and canr evaporate out if:

leakage, changes of contact potential and secondary emissionratio, and otherwise defective tubes.

Figs. l and 2 show, respectively, a portion of the surface of a AgMg strip or base member before any processing for 'use as a secondary emitter, and such a strip after preliminary processing with a low pressure of a molecular oxidizing gas to produce the golden MgO surface. The free Mg atoms have been shown schematically by stippling in Figs. 1 and 2. As indicated in Fig. 2, the formation of the MgO surface reduces the number of Mg atoms in the alloy to a small extent. In` accordance with the present invention, a silvermagnesium alloy secondary emitter in which the proportion of magnesium is small compared to the proportion -ofsilvery which has previously been heated in a suitable oxidizing atmosphere including a molecular oxidizing material which does not diffuse freely into the alloy, such as one containing water vapor to form a MgO secondary emitter surface, is reheated in an oxygen atmosphere, to render the emitter incapable of appreciable evaporation of magnesium, as shown schematically in Fig. 3. Experiments have shown that when a pre-oxidized AgMg sample is heated in oxygen, it becomes brittle, which means that the oxygen penetrates the already formed MgO film, and oxidizes free magnesium atoms left in the alloy, thus freezing them in place.

Measurements were made to determine the amount of magnesium evaporation at 800 C. from such a two-step activated sample by comparison with evaporation from samples of AgMg with known Mg concentration. The results show that when a water vapor activated sample is baked at 700 C. in oxygen at atmospheric pressure for 5 minutes by high frequency induction heating, the magnesium evaporation is only about one-twentieth that of the water vapor activated sample without the subsequent oxygen bake. Therefore, if the original alloy contained 1.7 percent magnesium, as in the alloy mentioned above, the iinal alloy contains not more than one-tenth of one percent by weight of free Mg atoms. Conditions can be set to freeze an even greater amount of the magnesium atoms in place by this method. If all of the magnesium atoms in the AgMg alloy containing 1.7 percent-magnesium were oxidized, the inal emitter would be an alloy of silver and magnesium oxide having a magnesium oxide surface and having a composition of approximately 98.3 parts or 97.23 percent of silver, 1.7 parts or 1.68 percent of magnesium, and 1.1 parts or 1.09 percent of combined oxygen. The small amount of free magnesium left in the emitter is indicated'by the sparse stippling in Fig. 3. Moreover, a significant increase in secondary emission ratio is observed by this treatment, probably because of the effect of the oxygen in the Mg() surface. Some free Mg in the MgO surface may be oxidized which would increase the mean free path for secondary electrons and give rise to a greater yield of secondary electrons. The oxygen itself may also act as a source of secondary electrons because of its valence band of six electrons.

The treatment of the pre-oxidized AgMg emitter with oxygen can be carried out at temperatures from about 500 C. almost up to the melting point of silver, i. e., approximately 960 C., for periods from tive minutes to a half hour or more, depending on the temperature and pressure. The pressure of the oxygen is not critical, and may be as low as a few millimeters of mercury and as high as several atmospheres. In general, the lower the temperature and pressure are, the longer will be the time required to oxidize substantially all of the free magnesium in the alloy. The diusion rate of oxygen into silver is proportional to the temperature and the external oxygen pressure.

, In Fig. 4, the lower curve shows the secondary emission ratio of a AgMg alloy secondary emitter which had beenprocessed in water vapor to produce the desired mamas I- MgO surface without any additional treatment. The upper curve shows the secondary emission ratio of a second emitter which had been processed tirst in water vapor and then heated in oxygen at atmospheric pressure for a half hour at 600 C. The two emitters were mounted in the same tube envelope and subjected to the same tube processing including cathode breakdown and exhaust. As shown in Fig'. 4 the oxygen processing produces a considerable improvement in the secondary emis sion ratio, in addition to minimizing the evaporation of metallic magnesium onto various tube electrodes and the tube envelope during activation and operation of the tube.

What is claimed is:

1. The method of preparing a secondary emitter, comprising the steps of: heating a base member of AgMg alloy, in which the proportion of magnesium is small compared to the proportion of silver, in a iirst oxidizing gas medium including a low partial pressure of a molecular oxidizing material selected from the group consisting of water vapor, alcohols, carbon dioxide and nitrogen pentoxide, to produce a secondary emissive surface of Mg() on said base member; and then heating said surface-oxidized base member in a second oxidizing gas medium the oxidizing agent of which is essentially oxygen for a period of time sutiicient to oxidize substantially all of the free magnesium left in said base member to minimizeevaporation of free magnesium from the emitter upon any subsequent heating thereof during use.

2. The method according to claim l, wherein the partial pressure of oxygen in said second gas medium is substantially greater than that of the oxidizing material in said iirst gas medium.

3. The method according to claim 1, wherein said first gas medium is substantially free of oxygen.

4. The method according to claim l, wherein said second gas medium is essentially oxygen.

5. The method of preparing a secondary emitter oomprising the steps of: heating a base member of AgMg alloy, in which the proportion of magnesium is small compared to the proportion of silver, in a iirst oxidizing gas medium including a low partial pressure of water vapor, -to produce a secondary emissive surface of MgO on said base member; and then heating said surfaceoxidized base member in a second oxidizing gas medium the oxidizing agent of which is essentially oxygen for a period of time sufficient to oxidize substantially all of rthe free magnesium left in said base member, to minimize evaporation of free magnesium from the emitter upon any subsequent heating during use.

6. The method according to claim 5, wherein the vapor pressure 0f said water vapor is from 105 to l0-2 mm. of mercury.

7. The method -according to claim 5, wherein said base member is heated, in the first step, in a gas medium including water vapor at a vapor pressure of about 5x10-4 mm. of mercury at a temperature of about 550 C. `for approximately a half hour.

8. The method vaccording to claim 5, wherein said base member is heated, in the second step, in oxygen at Ia pressure from a few millimeters of mercury to several atmospheres.

9. The method according to claim 5, wherein said base member is heated, in the second step, in oxygen with high frequency induction heating for about iive minutes at labout 700 C. at atmospheric pressure.

10. The method according to claim S, wherein said base member is heated, in the second step, in oxygen for about a half hour at about 600 C. at atmospheric pressure.

11. A secondary emitter electrode comprising a base consisting essentially of magneisum oxide and silverv andcontaining less than one tenth of one percent by weight of free magnesium, the proportion ot magnesium oxide being small comparedV to the proportion of silver-,lV

said base having a secondary emjssive surface of magnesium oxide.

12. A secondary emitter according to claim 1 1, wherein said surface has a thickness of about 1000 angstrom units.

13. A secondary emitter electrode comprising a base consisting essentially of about 3 percent magnesium oxide, 97 percent silver and less than one tenth `of one percent of free magnesium, by weight, and having a secondary emissive surface of magnesium oxide.

14. The method of preparing va secondary emitter comprising the steps of: heating a strip of AgMg alloy consisting of approximately 98.3 percent 'silver and 1.7 percent magnesium, by weight, at a temperature of about 550 C. for about one-half hour in an `oxygen-free gas medium including a partial pressure of water vapor at a vapor pressure of labout 5 104 mm. of mercury, to produce a golden surface of MgO on ysaid strip; `and then heating said oxidized strip with high frequency induction heating at about 700 C. in oxygen at atmospheric pressure for about five minutes, to oxidize substantially all of the free magnesium left in said alloy and thereby minimize evaporation of free magneisum from said emit lter on any subsequent heating thereof during use.

References Cited in the le of this patent UNITED STATES PATENTS 2,156,262 Fink et al. May 2, 1939 2,233,276 Zworykin et a1 Feb. 25, 1941 2,266,595 Fraenckel Dec. 16, 1941 2,373,937 Allen Apr. 17, 1945 2,393,803 Nelson Ian. 29, 1946 2,447,038 Spencer Aug. 17, 1948 2,477,279 Anderson, Jr. Aug. 26, 1949 2,586,771 Arditi et a1. Feb. 26, 1952 2,620,287 Bramley Dec. 2, 1952 OTHER REFERENCES Proceedings of the I. R. E., Vol. 38, pages 159-164, pub. 1950, page 160 is relied on. 

1. THE METHOD OF PREPARING A SECONDARY EMITTER, COMPRISING THE STEPS OF: HEATING A BASE MEMBER OF AGMG ALLOY, IN WHICH THE PROPORTION OF MAGNESIUM IS SMALL COMPARED TO THE PROPORTION OF SILVER, IN A FIRST OXIDIZING GAS MEDIUM INCLUDING A LOW PARTIAL PRESSURE OF A MOLECULAR OXIDIZING MATERIAL SELECTED FROM THE GROUP CONSISTING OF WATER VAPOR, ALCOHOLS, CARBON DIOXIDE AND NITROGEN PENTROXIDE, TO PRODUCE A SECONDARY EMISSIVE SURFACE OF MGO ON SAID BASE MEMBER; AND THEN HEATING SAID SURFACE-OXIDIZED BASE MEMBER IN A SECOND OXIDIZING GAS MEDIUM THE OXIDIZING AGENT OF WHICH IS ESSENTIALLY OXYGEN FOR A PERIOD OF TIME SUFFICIENT TO OXIDIZE SUBSTANTIALLY ALL OF THE FREE MAGNESIUM LEFT IN SAID BASE MEMBER TO MINIMIZE EVAPORATION OF FREE MAGNESIUM FROM THE EMITTER UPON ANY SUBSEQUENT HEATING THEREOF DURING USE. 